Field of the Invention
The present invention relates to a liquid developer for use in image-forming apparatuses that utilize an electrophotographic system, e.g., electrophotography, electrostatic recording, electrostatic printing, and so forth.
Description of the Related Art
Plate-based presses have in the past been used to produce printed material for which a certain number of copies are required, such as regional advertising, internally distributed corporate documents, and large posters. In place of these conventional presses, on-demand presses have entered into use in recent years; these on-demand presses can rapidly respond to a diversifying range of needs and support inventory reductions. Electrophotographic printers that use a dry developer or a liquid developer and inkjet printers capable of high speeds and high quality printing are anticipated for such on-demand printers.
Dry developers currently occupy the developer mainstream due to their handling advantages, which derive from the fact that a solid developer is being handled. However, viewed from the standpoint of preventing the image deterioration caused by changes in the environment, e.g., temperature and humidity, the environmental stability of the charging performance has been a problem with dry developers. In addition, the colored resin particles in a dry developer readily undergo aggregation during, for example, storage, and uniformity when the colored resin particles are dispersed has been a problem. In addition, with regard to their properties, when the colored resin particle diameter is made relatively small in pursuit of high resolution, the problems deriving from the fact that a powder is involved as described above become even more substantial.
Liquid developers, on the other hand, use an electrically insulating liquid as a carrier liquid and because of this are more resistant than dry developers to the problem of aggregation of the colored resin particles in the liquid developer during storage, and a microfine toner can thus be used. As a result, liquid developers provide a better fine line image reproducibility and a better gradation reproducibility than dry developers and are characterized by an excellent color reproducibility and also excellence in high-speed image-forming methods. Development is becoming quite active with regard to high-image-quality, high-speed digital printing apparatuses that exploit these excellent features by utilizing electrophotographic technologies that use liquid developers. In view of these circumstances, there is demand for the development of liquid developers that have even better properties.
A liquid developer production method has been disclosed in which, using a coacervation method, colored resin particles are dispersed in an insulating hydrocarbon dispersion medium in the presence of an acid group-containing resin and a compound that is the reaction product of a polyamine compound and a hydroxycarboxylic acid self-condensate (Japanese Patent No. 5,148,621). A liquid developer as disclosed in Japanese Patent No. 5,148,621 requires the removal of the electrically insulating liquid since a substantial deterioration in the appearance of the image ends up being caused when the electrically insulating liquid remains on the recording medium, e.g., paper or plastic film. A method generally used to remove the electrically insulating liquid has been the application of thermal energy to evaporate and remove the electrically insulating liquid, but this has not necessarily been desirable from an environmental standpoint, e.g., the vapor of a volatile organic solvent is emitted from the machine at this point and large amounts of energy are consumed.
To counter this, a method has been introduced in which the electrically insulating liquid is cured by photopolymerization. This photopolymerizable liquid developer uses a reactive functional group-bearing monomer or oligomer as the electrically insulating liquid, and a photopolymerization initiator is also added and dissolved thereinto. This photopolymerizable liquid developer is capable of high speeds because it is cured by a polymerization reaction induced by the irradiation of light, e.g., ultraviolet radiation, on the photopolymerizable liquid developer.
Such a photopolymerizable liquid developer has been disclosed in the form of a photopolymerizable liquid developer containing a toner particle that contains a rosin-type resin and is surface-modified by polyalkyleneimine, an insulating liquid comprising a liquid epoxy-modified compound, and a cationic photopolymerization initiator (Japanese Patent No. 5,277,800). However, there is a large amount of free polyalkyleneimine in the method disclosed in Japanese Patent No. 5,277,800, and this ends up reacting with the photopolymerization initiator. Due to this, inhibition of the polymerization of the polymerizable liquid monomer is prone to occur, and this makes it necessary to use very large amounts of the cationic photopolymerization initiator in order to obtain a satisfactory curability.